It is proposed to conduct further studies of the extra reactions of certain HL-A antisera with the cells of chronic lymphatic leukemia and possibly acute lymphatic leukemia. We have already shown that a complete iso-antigenic system not related to the HL-A system can be detected on CLL cells with these antisera. The next goal is to determine whether this iso-antigenic system is composed of leukemia-specific iso-antigens, or is a hyperexpression on the CLL cells of antigens determined by the MLC locus or perhaps immune-response genes or other cell surface factors not normally sufficiently expressed to be detected by standard cytotoxic methods. In addition: further delineation of the HL-A system and of sub-groups of various specificities within the system will be done. This will include the use of peripheral lymphocytes from normal persons and also lymphocytes which have been placed in tissue culture for varying periods of time, to study changes in cell surface antigenic makeup.